Eukaryotic cells rely on a conserved network of proteins to monitor the status of their genomes. This surveillance system detects double-strand breaks (DSBs) and transduces a checkpoint signal to the cell cycle machinery to coordinate cell division with DNA repair. Any defect in properly sensing and signaling aberrant DNA structures can lead to genome instability, which is a hallmark of cancer cells. This study will extend our efforts to use the fission yeast Schizosaccharomyces pombe to model mechanisms by which cells detect DSBs and activate a checkpoint response. The focus will be on two proteins: Crb2 and Ctp1. Crb2, an ortholog of the mammalian tumor suppressor/checkpoint protein 53BP1, belongs to a class of checkpoint proteins that also includes the tumor suppressor BRCA1, all of which have a C-terminal BRCT repeat. Like 53BP1 and BRCA1, Crb2 associates with DSBs, forming microscopically visible nuclear foci at sites of DNA damage. Ctp1, an ortholog of mammalian tumor suppressor/checkpoint protein CtIP, functions with the Mre11-Rad50-Nbs1 complex in resecting DSBs as the part of the initial step of DSB repair by homologous recombination. This study will: (1) examine the interactions between Crb2 and modified histones that define one mechanism of recruiting Crb2 to DSBs;(2) further characterize a mechanism responsible for histone modification- independent relocalization of Crb2 to DSBs that involves Cut5, the ortholog of human TopBP1;(3) define a mechanism in which fission yeast Tel1 (ortholog of human ATM tumor suppressor/checkpoint protein) activates Chk1 checkpoint kinase in the pre-resection phase of checkpoint signaling;and (4) determine how Ctp1 governs DSB repair during the cell cycle. Accomplishment of these aims will yield a more comprehensive mechanistic and functional understanding of the cellular response to DNA damage. A high-resolution picture of these mechanisms in fission yeast will aid in achieving a more detailed understanding of DNA damage responses and their connection to cancer in humans. PUBLIC HEALTH RELEVANCE DNA damage checkpoint proteins act as cellular firefighters: they rush to double-strand breaks in DNA where they help to repair the damage and send signals to stop cell division. This study will determine how one of these proteins, Crb2, is recruited to double-strand breaks. Insights gained from these studies will provide a more detailed understanding of DNA damage responses and their connection to cancer in humans.